1. Field of the Invention
This invention relates to improvements in a separation module and a bundle unit of hollow thread-type porous membrane elements and methods of producing the sames, and more particularly to such producing methods as to radially uniformly distribute the hollow thread-type porous membrane elements in the separation module and in the bundle unit to improve a separation efficiency of a separation module.
2. Description of the Prior Art
Separation modules of hollow thread-type porous membrane elements have been extensively used to separate or filter substances in the fields of electronic industry, food industry, medicine industry which require to the separation modules a low solubility and a high resistance to chemicals and steam during sterilization, because the porous membrane elements are made of high polymer materials. A typical separation module includes many U-shaped hollow thread-type porous membrane elements which are bundled and inserted in a cylindrical casing in such a manner that the porous membrane elements are bound to each other at their free end sections and fixed to the inner wall of the casing by using adhesive of polyurethane resin, epoxy resin or the like. The free end sections of the bundied porous membrane elements remain opened thereby allowing fluid to pass therethrough. With such a separation module, fluid to be filtered or to be subjected to separation passes through micro-pores formed in a generally cylindrical wall of each porous membrane element and flows through the above-mentioned open free end section of each porous membrane element to be discharged from the separation module.
In such a separation module, the hollow thread-type porous membrane elements are merely bundled randomly, and therefore clearances among adjacent thread-type porous membrane elements tend to be not uniform. This causes a so-called channel phenomena in which fluid to be treated flows concentrically through a location or passage having a low flow resistance during treatment of the fluid. Under this phenomena, there exit dead spaces or channels which do not function of separation or filtration, thus lowering a separating or filtering efficiency of the separation modules of the hollow thread-type porous membrane elements. In order to improve the separation modules in view of the above problems, it has been proposed to blow air from a direction to an end face of the bundled porous membrane elements. However, this method is insufficient to uniformly distribute the porous membrane elements so as to be not only difficult to provide high quality separation modules but also provide a disadvantage to damage the porous membrane elements.
Additionally, since the adhesive such as polyurethane resin or epoxy resin is used to fix the bundled hollow membrane elements in the casing, there are much dissolved substances from the adhesive, and low in resistance to heat and chemicals. As a result, the separation module have fitted to only a limited use such as water purification. In place of such a method of using adhesive, there is welding or melting method in which thermoplastic resin is filled in clearances among the adjacent hollow thread-type porous membrane elements, and then the thermoplastic resin is molten under heating to bind the porous membrane elements to each other. However, in such a method, it is difficult to maintain the generally cylindrical shape of each hollow thread-type porous membrane element during melting of the thermoplastic resin under heating, thereby raising problems in which the porous membrane elements are broken while changing the inner and outer diameters of each hollow thread-type porous membrane element.
In view of the above, the following production method of the separation modules has been proposed and is disclosed in Japanese Patent Provisional Publication No. 1-293105: First, the inside of each thread-type porous membrane element is filled with a filler material which exhibits a fluidity during heating to melt thermoplastic resin, and then the thermoplastic resin is filled to clearances among the porous membrane elements. Thereafter, heating is made to melt the thermoplastic resin thereby to bind the porous membrane elements to each other. Finally, the filler material is removed from the inside of each porous membrane element. More specifically, a thermally contractible tape is wound on the outer peripheral surface of a bundled body of the thread-type porous membrane elements with an auxiliary member filled among the porous membrane elements. As a result, the molten auxiliary member can fill the clearances among the porous membrane elements while the bundled body of the porous membrane elements are clinched under the contraction of the thermally contractible tape, thereby obtaining a fluid-tight seal among the porous membrane elements.
However, in the separation module made by this production method, it is required to make an operation of winding the tape around the bundled body of the porous membrane elements. Additionally, since the tape is freely wound, the outer diameter of the bundled body with the tape cannot become constant or take a predetermined value throughout many products. The bundled body with the tape is used, as a separation or filtering element, to be disposed in a casing, and therefore an O-ring or a packing is required between the outer peripheral surface of the separation element and the inner peripheral surface of the casing, or otherwise the auxiliary member is again required to be filled and molten therebetween. Consequently, in case of using the packing or the like, this separation module has encountered the problems of liquation of the material of the packing into the liquid to be subjected to separation, and a low sealing ability between the packing and the bundled body of the porous membrane elements owing to non-uniform outer diameters of the bundled body of the porous membrane elements (or products). In case of using again the auxiliary member, the separation module has encountered problems of failed binding of the outer peripheral portion of the bundled body, a failed sealing due to remelting of the auxiliary member which has been previously filled and molten among the porous membrane elements, and an increased seep number in the production process.
Another production method has been proposed and is disclosed in Japanese Patent Provisional Publication No. 4-63117 in which a separation unit of hollow thread-type porous membrane elements is produced by using polyolefin resin as the material of the porous membrane elements and of a sealing material for binding the end sections of the porous membrane elements. The production method is as follows: First, a suspension liquid of polyolefin resin fine powder (the sealing material) is prepared. The end sections of bundled porous membrane elements are dipped in the suspension liquid. Then, the liquid of the suspension is evaporated while putting the polyolefine resin fine powder in a molten state under heating at a temperature higher than the melting point of the sealing material. Thereafter, slow cooling is made to an ordinary temperature thereby to solidify the sealing material. In this production method, no operation of filling the spaces in the sealing material fine power is made at the step of heating, and therefore much bubbles remain in the sealing material. As a result, the separation module produced by this method is low in initial sealing ability and in durability particularly at a high temperature condition and at a condition in which a pressure is repeatedly applied.